The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
The separation of wanted (target signal, S) and unwanted (noise signal, N) parts of a sound field is important in many audio applications, e.g. hearing aids, various communication devices, handsfree telephone systems (e.g. for use in a vehicle), public address systems, etc. Many techniques for reduction of noise in a mixed signal comprising target and noise are available. Focusing the spatial gain characteristics of a microphone or a multitude (array) of microphones in an attempt to enhance target signal components over noise signal components is one such technique, also referred to as beam forming or directionality. [Griffiths and Jim; 1981] describe a beamforming structure for implementing an adaptive (time-varying) directional characteristic for an array of microphones. [Gooch; 1982] deals with a compensation of the LF roll-off introduced by the target cancelling beamformer. [Joho and Moschytz; 1998] deals with a design strategy for the target signal filter in a Griffiths-Jim Beamformer. It is shown that by a proper choice of this filter, namely high-pass characteristics with an explicit zero at unity, the pole of the optimal filter vanishes, resulting in a smoother transfer function.
WO2007106399A2 deals with a directional microphone array having (at least) two microphones that generate forward and backward cardioid signals from two (e.g., omnidirectional) microphone signals. An adaptation factor is applied to the backward cardioid signal, and the resulting adjusted backward cardioid signal is subtracted from the forward cardioid signal to generate a (first-order) output audio signal corresponding to a beam pattern having no nulls for negative values of the adaptation factor. After low-pass filtering, spatial noise suppression can be applied to the output audio signal.
The present disclosure relates to an alternative scheme for implementing a beamformer.